This invention relates to submarine coaxial cables.
There is presently manufactured submarine coaxial cables comprising a central strength member sheathed in copper to act as an electrical conductor, polyethylene insulation surrounding this inner conductor, an outer return conductor surrounding the insulation, and finally, an outer polyethylene sheath extruded around the return conductor.
The return conductor is formed from a metal tape (e.g. copper) folded around the insulation with a longitudinal seam where the longitudinal edges of the tape overlap. The overlap of the edges of the tape is typically about one quarter of an inch for 1.47 inch diameter submarine cable, or about 5% of the return conductor circumferential length. This return conductor does not act as an efficient screen owing to the "leakage" which will occur along the seam because the overlapping edges are not galvanically connected otherwise than as may occur as a result of simple abutment together.
For the major part of the length of a submarine cable transmission system the return conductor is not required to screen the inner conductor from interference et cetera because no interference is likely to occur at the water depth encountered. However, toward the terminal ends of the system on land and where it approaches the land in shallow water, screening is desirable.
The conventional screening technique comprises helically winding six very thin mild steel tapes with a long lay (approximately 12 inches) and with a helical gap. This winding is bound by a thin copper tape about 2 inches wide and also helically wound. Then a proofed cotton tape which acts as a bedding is wound on, and a single steel tape is applied longitudinally with a gap. Then soft steel tapes are helically wound first right then left then right then left. The whole is sheathed in polythene and the normal wire armouring, if necessary, is applied comprising steel wires and jute bedding. The aforementioned screening technique is obviously expensive, and substantially increases the cable size and weight.
The purpose of the present invention is to provide a cable screening technique which is less expensive, simpler and smaller than the present technique.